In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of a Device-to-Device (D2D) proximity service is discussed as a new function on and after Release 12 (see Non Patent Document 1).
The D2D proximity service (D2D ProSe) is a service enabling direct device-to-device communication within a synchronization cluster including a plurality of synchronized user terminals. The D2D proximity service includes a discovery procedure (Discovery) in which a proximal terminal is discovered and D2D communication (Communication) that is direct device-to-device communication.
Further, a discovery procedure in which a user terminal that exists in a certain cell (serving cell) discovers a proximal terminal that exists in other cell (neighbor cell) is called an inter-cell discovery procedure (Inter-Cell Discovery). Further, D2D communication in which a user terminal that exists in a serving cell performs communication with a proximal terminal that exists in a neighbor cell is called an inter-cell D2D communication (Inter-Cell Communication).